To put a hard, thin, wear and fatigue resistance layer on steel parts which have been quenched and tempered, a common processing technique is employed called "low temperature nitriding". This is a process which requires special heat treating furnace equipment in order to carry out nitriding at a temperature in the range of 925.degree.-1050.degree. F. The process sequence consists essentially of: (a) roughly shaping the part to be treated, often by hot forging, (b) hardening by austenitizing, quenching and tempering to a hardness usually in the range of R.sub.c 25-35, (c) finish machining, and (d) nitriding at 925.degree.-1050.degree. F. Because the nitriding temperature is about the same as or lower than the tempering temperature, the hardness of the core material can be maintained in the range of R.sub.c 25-35. Such a process is used commercially for making some automotive-type gears.
Other currently used commercial nitriding methods either require the use of a molten salt bath (necessitating special control of bath composition) or use of a furnace with special equipment to contain an ammonia-bearing gaseous atmosphere. These processes are detailed in the eighth edition of Metals Handbook, Volume 2. The salt bath technique, as well as the technique with a furnace having an ammonia-bearing atmosphere, undesirably require considerable care to regulate the nitrogen potential so that results are consistent. It is also apparent that with any of the above processes, special equipment and considerable manipulation are required to obtain good results. This is not always obtainable in certain plant situations and certainly it is desirable to reduce the required skills in obtaining good results with any procedure.
Thus in plants or heat treating shops which lack the special equipment for low temperature nitriding or the special equipment required for salt bath or gas atmosphere nitriding, it would be most advantageous to have a method for pack nitriding, as convenient as pack carburizing. However, it is important to keep in mind that the science of pack carburizing is non-analogous, except as to cost and convenience, because it is carried out at temperatures of 1500.degree. F. and above, where low alloy steels are austenitic, and is followed by quenching and tempering.